1. Field of the Invention
The present invention relates to a developer carrying member used in image forming apparatus such as electrophotographic recording apparatus and electrostatic recording apparatus. More particularly, it relates to a technique for surface modification of a developer carrying member used in a developing device.
2. Related Background Art
The methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748, etc. are hitherto known as electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member, utilizing a photoconductive material as a photosensitive layer and according to various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing of the toner image by the action of heat, pressure, heat-and-pressure, or solvent vapor.
Various processes are also known in which an electrostatic latent image is formed into a visible image by the use of a toner.
For example, known development processes include magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, cascade development as disclosed in U.S. Pat. No. 2,618,552, powder cloud development and fur brush development as disclosed in U.S. Pat. No. 2,221,776, and liquid development.
In these development processes, dry development, in which a toner is used in a powdery state, is widely put into practical use in view of its readiness in handling of a developer.
As a developer carrying member used in the dry development, Japanese Patent Application Laid-open No. 57-66455 discloses an example. It is known to mold a metal such as aluminum, nickel or stainless steel, or an alloy compound thereof, into a cylindrical form and treat its surface by electrolysis, blast finishing or by means of sand paper or the like so as to have a given surface roughness.
Such a developer carrying member is inexpensive and can give a relatively stable and high-quality image, but on the other hand makes it difficult to control the static charge of toner when a one-component type developer is used in which static charge is imparted from a developer carrying member. Although various approaches to improvement have been made from the direction of developers, the problem concerning a static charge non-uniformity has not been completely settled.
As disclosed in Japanese Patent Application Laid-open No. 61-180267, it has been proposed to coat the surface of a developer carrying member with a conductive film-forming composition containing a texture-forming agent or constitute a developer carrying member with the same material as a film-forming composition.
In these methods, however, the problems have not been well settled with respect to one-component type magnetic developers.
The reasons therefor are as follows. Because of the developer contains a substance such as a magnetic material having a relatively low resistance, charges tend to slip away and the static charge tends to become non-uniform. Also, because the developer contains an inorganic material such as a magnetic material with a high hardness, the abrasion of a coating film is accelerated. Thus, it is difficult to stabilize image quality.
The above phenomenon is particularly remarkable in a production process in which a coating film is formed using a liquid or pasty coating composition, as is seen in the process disclosed in Japanese Patent Application Laid-open No. 52-119651.
In the case of the liquid or pasty coating composition, such a phenomenon is due to the fact that there is a period of time during which a pigment is movable through the inside of the coating film (i.e., a tack-free time) and hence the surface of the developer carrying member tends to become smooth because of the surface tension or the compatibility of materials.
In Japanese Patent Application Laid-open No. 60-80876, it is proposed to coat the surface of the developer carrying member with a film-forming composition having a conductivity or constitute a developer carrying member with the same material as a film-forming composition.
This method also, however, has not achieved sufficient image stability to duration of copying on a large number of sheets. As a durability test proceeded, it was seen that image density rose (became higher) or fell (became lower) and thus the image density was not stable.
It is presumed that this has been caused by a change of the state in which a pigment having a conductivity projects on the surface of the coating film.
The projection of the pigment is relatively small because of the surface tension of materials and the compatibility of materials when the developer carrying member is in the initial state. As the durability test proceeds, however, the surface layer of the developer carrying member is scraped by a developer, resulting in the formation of a new surface. This is presumed to be the reason. On the other hand, when a substance having a cleavability as exemplified by graphite is used as the pigment, it is seen that the above phenomenon occurs less. This is presumed to be due to the fact that the cleavability of the substance immediately stabilizes the state of the surface.
When, however, graphite is added, the following problems arise.
(1) Since graphite is usually scaly, even a material with an average particle diameter of several microns comprises a particle with a width of several tens of microns in the direction of the major axis (i.e., the direction of the cleavage surface). Even when the ratio of a conductive surface (a pigment surface) to an insulating surface (a resin surface) is in a stable state from a macroscopic viewpoint on the surface of developer carrying member, the ratio is non-uniform from a microscopic viewpoint (i.e. at size level of a developer) and hence the ability of a developer carrying member to impart static charge to a toner becomes non-uniform. This causes a local change of thickness of a toner coat, resulting in a change of density. PA1 (2) Since the cleavage surface is flat, the phenomenon of toner adhesion tends to occur.
The above phenomenon particularly occurs in the production process in which a coating film is formed using a liquid or pasty coating composition according to the method as disclosed in Japanese Patent Application Laid-open No. 52-119651.
In these methods, such a phenomenon is due to the fact that there is a period of time during which pigment in a liquid or pasty coating composition, is movable through the inside of the coating film (i.e., a tack-free time) and hence the surface of the developer carrying member tends to render the surface of a binder resin because of surface tension or compatibility of materials.